1. Field of the Invention
The present invention relates to a circuit for providing a reference signal that can be used in conjunction with a circuit for signal processing, and that provides a way for the reference generating circuit to monitor itself to determine when the reference voltage is stable.
2. Description of Related Art
Bandgap based voltage and current generation circuits have been widely used to provide bias currents and reference voltages for analog and mixed signal integrated circuits. The circuits that receive bias currents and reference voltages often perform important signal processing functions and make decisions that can affect the overall operation of an integrated circuit. It is extremely important that the decisions not be affected by undesired fluctuation in the bias current or reference voltage. At the start-up of a semiconductor chip, the circuits that receive and rely upon the bias current or reference voltage can reach operable states before the bias current or reference voltage reaches it steady-state value. If this occurs at a point in time when the value of the bias current or reference voltage is unstable and at a small fraction of its steady-state values, the decisions made by the circuits that depend upon the bias current or reference voltage may be unpredictable. In order to prevent the unpredictable decisions from corrupting the chip function, the circuit that generates the bias current or reference voltage should output a signal indicating that the bias current or reference voltage has reached a condition necessary for proper operation of the chip.
One example of a prior art approach to providing a bandgap based reference voltage is disclose in U.S. Pat. No. 5,610,506, VOLTAGE REFERENCE CIRCUIT, (Issued Mar. 11, 1997). The ""506 patent proposes a circuit that indicates that the voltage generated by the bandgap circuit is not valid and this information is used to enable or disable the other circuits of the chip.
The approach taken in the ""506 patent concentrates on determining whether or not the supply voltage has reached a predetermined value. The focus of trying to determine whether or not the supply voltage has reached a specific value does not address the issue that arises as integrated circuits are powered by battery cells where the supply voltage cannot be assumed to be a fixed or certain value. Also, in order to maximize the operation of chips, circuits should be allowed to operate over as wide a range of power supply voltage as is possible. Thus, there is a need for generating a signal that indicates the proper operation of a bandgap circuit as soon as the generated voltage or current is stable enough to provide a bias current or reference voltage.
The method and apparatus of the invention provide a circuit that generates a reference voltage. The circuit also provides nodes that produce test voltages. These test voltages are interrelated and also relate to the state of the reference voltage. By comparing the test voltages, the circuit is able to make a determination as to when the reference voltage has reached a ready state. An important feature of the invention is that the test voltages are derived from the same circuit that generates the reference voltage. Thus, the determination that the reference voltage is suitable for use is made as quickly as possible, because the voltages being compared are part of the reference voltage circuit itself and are not derived from some other source.